Just sold mine for pure, providing a little proof. Bit of an ugly range, but I don't see a good way around it.

1. Sale 1, ~16 keys: offered with 8 keys on Blizzardy Front Runner: http://www.tf2outpost.com/trade/26011144

The seller had it listed for 17 pure: http://www.tf2outpost.com/trade/26262541

Seller, before: http://backpack.tf/profiles/76561198119597043?time=1434920692

Seller, after: http://backpack.tf/profiles/76561198119597043?time=1435042800

Seller appears to have been counting it as 16 keys or so. The math: Front runner (~22.5, nearly outdated) minus keys = 14.5, divide by 0.9 to apply overpay with the offered greased, value comes out to 16 and change. Looks like a trader that just checked backpack.tf and offered based on it. The person who accepted it clearly values the hat higher, and rejected an offer of 17 pure: http://www.tf2outpost.com/trade/26401656

I'm calling this one an outlier.

2. Sale 2: 27 keys. Listed by me, buyout 26 on outpost: http://www.tf2outpost.com/trade/26272321

Still had it as 27 on backpack because I forgot to update it, and ended up selling for 27 pure:

(Inventory history; I had several other keys coming and going, backpack.tf would need key histories):

http://i.imgur.com/05QHjDS.png

http://i.imgur.com/y40EMAL.png

3: Sale 3: 18 keys, what I bought it for back in mid-May:

http://www.tf2outpost.com/trade/25723634

Buyout was 22 keys, bought it for 18 pure:

http://i.imgur.com/B5lPrw1.png

http://i.imgur.com/YvCBAkF.png

Only current listing:

Buyout 25 keys, offer of 17 pure rejected:

Taking 18 as the low-end from my pure buy and a rejected lower offer in pure. Using 25 as the ceiling based on the buyout, and that I probably would have taken 25 myself.

Feedback (such as any other sales for unusuals) would be great, feel free to counter if you find a couple that don't fit here.